Hello mate - my old Zafira was loosing water at an alarming rate but with no visible signs of it being used by the engine. Upon further investigation, I found a wet patch on the underside of the passenger side floor pan. Turned out, the O rings that seal the coolant pipes into the heater matrix had turned rock hard and were leaking coolant all down underneath the passenger carpet. Because the carpets have sound proofing etc, there was no evidence from the top that anything was leaking underneath. It was quite a job to get the centre console shifted and all the bits of trim but once the heater matrix was exposed, it was obvious where the leak was coming from. Getting the matrix out was tricky, but we managed it and then re-ringed the pipes and put it all back together - no more leaks !!
On inspection, the O rings were quite brittle as if they were 'calcified' or covered in salt deposits from the water. I wonder if this might occur if the antifreeze level in the water is too low ? Just guessing..